Linear actuators are employed in many and diverse environments. For many applications it is preferred that the linear actuator be unaffected by external shocks. A common method for limiting the effect of external mechanical shocks acting upon a linear actuator is to use a strong return spring that holds a plunger of the actuator in position up to a certain level of acceleration. Typically such strong return springs are either compression springs or conical springs. A major disadvantage of this strong return spring approach is that the strong return spring requires the actuator to have enough performance to overcome the return spring. The power necessary to achieve this spring-overcoming performance may be larger than necessary to move the actuator, and the larger power may in turn problematically increase heat in the actuator. An additional disadvantage is that these devices with higher return spring and actuation forces also have a significant increase in undesirable audible noise.
Some conventional actuators, represented by the actuator of FIG. 15, employ an internal lock spring, separate from the return spring, to stop against a ledge in the bobbin to prevent motion of the actuation pin (plunger) until power is supplied. Due to the short lever arm on the lock spring, the spring rate is higher in order to return the lock spring to the lock position. The actuator of FIG. 15 also involves closing air gap solenoid construction, where a base is axially in line with the actuation pin (plunger). With this construction, when the lock spring is actuated to the base, there is a frictional drag that needs to be overcome. As power increases, the plunger can be attracted to the base before the lock spring moves away from the lock position which would cause the unit to fail to actuate. Therefore extra power is needed to make sure the lock spring moves first. Since this type design utilizes a closing air gap and allows the plunger to contact the base, noise and residual magnetism are a concern. If the lock spring is made too weak, since it contacts the base, residual magnetism is of concern here as well. In addition, the residual magnetism concern also results in higher levels of return spring force being needed.